Aspects of the invention relate to an aircraft wing. In particular, aspects of the invention relate to an aircraft wing having a system or device for establishment of laminar boundary layer flow on said wing.
The wing of a conventional aircraft has movable devices called slats positioned along the leading edge of the wing to improve high lift performance. The slats are retracted in the cruise phase of the flight. At the leading edge of an aft swept wing—which is typical for transonic aircraft—there is a spanwise flow from wing root to tip above which the air flows over the upper surface and below which the air flows over the lower surface. This flow is known as the attachment line flow. The presence of the leading edge slats or other types of movable high lift devices, e.g. droop nose devices, introduces surface discontinuities (steps and gaps) on the leading edge in the spanwise direction. The presence of these discontinuities means that the spanwise attachment line flow will be turbulent (rather than laminar) in the cruise flight phase.
Up to now, aircraft slats are designed so that in a slat-to-slat junction these surface discontinuities are as low as possible and that a smooth leading edge is provided. For example, slat seals are used in a junction area between adjacent slats.
It is known in the art that turbulences at the attachment line flow can disturb a boundary layer flow over an airfoil. It is especially referred to U.S. Pat. No. 3,288,399 describing an arrangement in which a shaped “bump”, having a bluff front end and an inclined rearward surface, is fitted to the leading edge. The bluff front creates a stagnation region whereby a laminar boundary layer is established on the rearward surface. Successful relaminarisation of a turbulent attachment line flow has been demonstrated using such known “bump” device in both wind-tunnel and flight experiments. However, such a bump only works provided the attachment line position on the leading edge is fixed during the cruise. This is the case for a fin, which has been shown in a laminar flow flight test using Airbus A320, but not for a wing, as the attachment line position changes with aircraft lift and aircraft weight.
Laminarisation of a turbulent leading edge flow can also be achieved by surface suction to a porous skin. However, porous surfaces are expensive to manufacture, have complications relating to cleaning and icing, and may have a weight penalty due to the required pipe-work. Examples for such active suction systems can be found in U.S. Pat. No. 6,216,982 B1 and EP 1 744 952 B1. Such suction systems normally are active systems that need a further pump or suction mechanism.
EP 2 091 814 B1 relates to a system for establishment of laminar boundary layer flow on an airfoil body which system is a passive system without use of additional pump means. This proposes a kind of “bleeding slot”. It is proposed to mount a plate to the leading edge. This plate forms—together with the airfoil leading edge region—a duct having a duct entrance for receiving spanwise flow along the leading edge of the airfoil body; a duct entrance extends around the leading edge and over the range of positions of the attachment line. Along the spanwise edges of this plate, duct exits are formed. Hence, this plate functions in flight such that attachment line flow air is drawn through the duct entrance and led upwards or downwards. Hence, a turbulent attachment line flow can be relaminarized.
Such a kind of plate as known from EP 2 091 814 B1 has been successfully tested; however, there are concerns about the integration of this plate onto the leading edge, its tolerance to damage, and the impact of the device on ice growth.